October 2016

So, I ordered the parts last weekend and over the course of the last week, they arrived, in three dispatches.

Might’ve been cheaper for RS to wait for the lot to arrive, I’d have been happy for them to do that had they asked, but never mind, they all turned up. So today, in clouds of solder smoke out on the back deck, I set to work soldering up the first of these boards.

At first, I just put all the SMD parts on, the programming header, the 5V reg, and the DC input lead. This proved a little fun. When I ordered the parts, I had initially put a lot of resistors, all 0805 size, to fill some of my stock. Then I took some off the list because I was over budget a little. Murphy dictates the ones that you take off are the ones you need.

I had thought I had spares anyway… turns out those were 1206s. And, in a pinch, one can use 1206 size resistors on an 0805 size footprint, or at least the “hand-soldering” variants that Kicad produces. The 10k pull-up resistors used on the MOSFETs and the 47k pull-up on the reset line are all 1206s, and they fit okay.

Had I remembered I had 1206s not 0805s, I’d have used 1206s. They’re a little friendlier to work with.

Lessons:

• always check the list before hitting the order button!
• In a pinch, you might be able to squeeze a 1206 onto an 0805 footprint, but don’t bet on it!

The other thing I could have done better would be to nudge the footprint for the 7805 along a little so that the switch-mode alternative part (ROHM BP5293-50) would fit. Better yet would be to not be so darn lazy and actually make a footprint for it. It’s a little cosy, but it snuggles up nicely against the capacitor.

Similarly, slightly bigger holes for the power connectors would help too, although the pins on the MOSFETs aren’t exactly big and they’re supposed to handle 70A. The lead length is short though, so not much resistance, I’ll probably get away with it.

The LEDs also proved a challenge, namely figuring out which way around they went. Most were all Osram CHIPLED parts, had a milky-white lens, and no markings visible on the top (one nondescript marking on the bottom), so it was difficult to tell anode from cathode. I wound up using my electronics kit to supply 4.5V through a 1k resistor out to two wires (a twisted pair from some CAT5e) which I could touch to each side to know which was which.

The one exception was the batch of yellow LEDs I bought, Osram didn’t seem to do the yellow ones so wound up buying Kingbright ones, which featured two green dots showing the cathode (the “bar” end of the schematic symbol). When I run out of the existing stock of red, orange and green LEDs, I’ll get more like these I think as it makes life much easier.

This was shortly after programming, the code running on the board for the first time.

Seeing the LEDs blinking, I disconnected the board and proceeded to fit the remaining through-hole components. The resulting board seems to be making the right noises, turning the fan on and off in response to the internal temperature sensor, and running from a 9V source.

Now I guess I’ll wire up the remaining leads, hook it to DC power and see what happens.